ANATOMY AND PHYSIOLOGY OF PREGNANCY

anatomy and physiology of pregnancy

 

Anatomical Changes

During pregnancy, women undergo many physiological changes to ensure that the needs of the fetus are met.

 

•Pregnancy causes physiological and homeostatic mechanisms to ensure proper fetal development. Increases in blood sugar, breathing, and cardiac output are required. Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and the menstrual cycle.

 

•As the fetus grows and develops, several anatomical changes must occur to the female body to accomodate the growing fetus, including placental development, weight gain and abdominal extension, breast enlargement and glandular development, and posture changes.

 

•During the first trimester,fertilization and implantation occur, followed by the embryonic stage, and concluding with the beginning of the fetal stage when most of the body systems develop.

 

•During the second trimester, morning sickness subsides, the uterus expands up to 20 times its normal size, breasts enlarge, and movement of the fetus may be felt.

 

•During the third trimester, final weight gain occurs as the fetus grows most rapidly during this stage. The belly drops and fetal movement can become quite strong.  The woman feels ready to give birth.

 

implantation

The attachment of the fertilized ovum to the uterus wall.

 

 

pelvic cavity

 

The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor.

decidua

A mucous membrane that lines the uterus: it is shed during menstruation and modified during pregnancy.

 

Examples

 

A third trimester baby born prematurely may survive. The use of modern medical intensive care technology has greatly increased the probability of premature babies surviving and has pushed back the boundary of viability to much earlier dates than would be possible without assistance. In spite of these developments, premature birth remains a major threat to the fetus, and may result in ill health in later life, even if the baby survives.

 

Changes during pregnancy

 

 During pregnancy, the woman undergoes many, normal physiological changes. These cardiovascular, hematologic, metabolic, renal, and respiratory changes become very important in the event of complications. The body must change its physiological and homeostatic mechanisms to ensure the fetus develops properly. Increases in blood sugar, breathing, and cardiac output are required. Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and the menstrual cycle.

 

 Pregnancy begins when the developing embryo implants in the endometrial lining of a woman's uterus. Most pregnant women do not have any specific signs or symptoms after implantation, although it is not uncommon to experience minimal bleeding. After implantation, the uterine endometrium is called the decidua. The placenta, which is partly formed from the decidua and partly from outer layers of the embryo, connects the developing embryo to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. The umbilical cord connects the embryo or fetus to the placenta. The developing embryo undergoes tremendous growth and changes during the process of fetal development.

 

The first trimester

 

 The first 12 weeks of pregnancy is known as the first trimester. During this trimester, fetal development can be divided into different stages. Fertilization occurs during the 3rd week, while implantation occurs during the 4th week. At this point, the zygote becomes a blastocyst and the placenta starts to form. The 5th week marks the beginning of the embryonic stage which lasts for about two months, at which point the fetal stage begins. At the beginning of the fetal stage, the risk of miscarriage decreases sharply. All major structures, including the head, brain, hands, feet, and other organs have been formed. Once pregnancy moves into the second trimester, the risks of miscarriage and birth defects drops drastically.

 

The second trimester

 

 Weeks 13 to 28 of the pregnancy is called the second trimester. Most women feel more energized in this period; they put on weight as the symptoms of morning sickness subside and eventually fade away. By the end of the second trimester, the expanding uterus has created a visible "baby bump". Although the breasts have been developing internally since the beginning of the pregnancy, most of the visible changes appear after this point. The uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy. Although the fetus begins to move and takes a recognizable human shape during the first trimester, it is not until the second trimester that movement of the fetus, often referred to as "quickening", can be felt. This typically happens in the 4th month, more specifically in the 20th to 21st week, or by the 19th week if the woman has been pregnant before. However, it is not uncommon for some women not to feel the fetus move until much later. The placenta fully functions at this time, and the fetus makes insulin and urinates. The reproductive organs distinguish the fetus as male or female.

 

The third trimester

 

 Final weight gain takes place during the third trimester, which is the most weight gain throughout the pregnancy. The fetus will be growing the most rapidly during this stage, gaining up to 28g per day. The woman's belly will transform in shape as the belly drops due to the fetus turning in a downward position ready for birth (Figure 0). During the second trimester, the woman's belly would have been very upright, whereas in the third trimester it will drop down quite low, and the woman will be able to lift her belly up and down. The fetus begins to move regularly, and is felt by the woman. Fetal movement can become quite strong and disruptive to the woman. The woman's navel will sometimes become convex, "popping" out, due to her expanding abdomen. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and backache. Movement of the fetus becomes stronger and more frequent. With improved brain, eye, and muscle function, the fetus is prepared for ex utero viability. The woman can feel the fetus "rolling"; it may cause pain or discomfort when it is near the woman's ribs and spine.

 

 There is head engagement in the third trimester: the fetal head descends into the pelvic cavity so that only a small part (or none) of it can be felt abdominally. The perenium and cervix are further flattened, and the head may be felt vaginally. Head engagement is known as the "baby drop": it releases pressure on the upper abdomen and leads to renewed ease in breathing. However, it severely reduces bladder capacity, increases pressure on the pelvic floor and the rectum, and the mother may experience the sensation that the fetus will "fall out" at any moment.

 

 

 

 Pregnancy causes major physiological changes to the mother’s body. An understanding of these changes is the key to safe obstetric anaesthesia. The causes of these changes are initially hormonal; metabolic and mechanical factors also contribute as the fetus develops.

 

Endocrine Function

 

The principle hormone of pregnancy is progesterone. Soon after fertilisation, the developing placenta produces human chorionic gonadotropin (hCG). This sustains the corpus luteum until the 6th to 8th week of pregnancy enabling it to produce progesterone. After this time, the placenta takes over as the main source of progesterone. The placenta also produces human placental lactogen. This has effects similar to growth hormone and may be the cause of maternal insulin resistance. It may also result in anatomical changes similar to acromegaly that are occasionally seen during pregnancy.

The thyroid gland undergoes hypertrophy during pregnancy. Increased production of thyroxine and triiodothyronine are normally balanced by increased production of thyroid binding globulin so the levels of the free hormones remain the same. Levels of parathyroid hormone tend to fall during pregnancy as does the level of serum calcium, although the level of ionized calcium remains constant due to the changes in the serum albumin concentration.

Due to the demands of the developing fetus metabolic rate increases in pregnancy. On average, 12kg weight is put on in pregnancy. This is accounted for by increases in the volumes of the plasma, interstitial fluid and amniotic fluid, as well as uterine growth, and of course the fetus. Increases in fat deposition are variable and may be used as an energy store for breast feeding in the post partum period.

 

 

 

 

Respiratory system

Major physiological and anatomical changes occur in the respiratory system during pregnancy due to a combination of both hormonal and mechanical factors. Dyspnoea is a common complaint in pregnancy affecting over half of women at some stage.

 

Difficult intubation is said to be very much more common in the pregnant patient at term. Firstly, due to increased breast size, insertion of the laryngoscope may be difficult (a laryngoscope with an angled blade may be useful). Secondly, airway mucosal oedema (which tends to be even worse in the presence of pre-eclampsia) may make the view at laryngoscopy poor. It is recommended that a smaller size endotracheal tube is used.

 

Nasal congestion can occur and nasal intubation is not recommended as it can result in trauma to the airways.

 

Hyperventilation occurs which is due both to an increase in tidal volume (40%) and a lesser increase in respiratory rate (15%). This results in a slight drop in the partial pressure of carbon dioxide (to approximately 32 mmHg or 4.3 kPa) resulting in a mild respiratory alkalosis (pH 7.44) Both the metabolic demands of the fetus and the increased work of breathing result in an increased oxygen consumption (up to 60 % during labour).

 

Functional residual capacity decreases by about 20 % (decreasing even further in the supine position).

 

The above changes in the respiratory system warrant a heightened awareness of the following factors when performing general anaesthesia.

1. Difficult intubation and a need for a range of ETT sizes.

2. Pre oxygenation is essential due to rapid de-saturation owing to an increased O2 consumption and a reduction in FRC

3. Maintenance of the ‘normal’ lowered levels of arterial PCO2 during mechanical ventilation

4. Reduced compliance necessitating higher airway pressures to maintain adequate ventilation

 

Cardiovascular system

 

Cardiac output increases measurably by about 5 weeks after conception. This occurs from a combination of increased stroke volume, increased heart rate and a decrease in total peripheral resistance. Cardiac output is increased by approximately 40% at the end of the first trimester. Anatomically the heart is displaced upward and to the left by the gravid uterus. Flow murmurs are quite common due to increase plasma volume and cardiac output. Regurgitant murmurs are also not unusual due to cardiac dilatation.

 

The ECG reflects these changes including, left axis deviation, ST segment depression and T wave flattening.

 

Systemic vascular resistance (SVR) occurs due to the vasodilatory effects of progesterone and the proliferation of low resistance vascular beds in the inter-villous spaces of the placenta. Blood flow to the uterus increases to about 700ml/min by term. Blood flow to the kidneys and skin also increase whereas flow to the brain and liver remain unaltered.

 

Blood pressure generally falls despite the increased cardiac output due to the lower SVR. The diastolic pressure tends to fall more than systolic pressure, the maximum decline around mid gestation. Diastolic pressure may fall as much as 20 % and systolic by around 8 %

 

 

 

Aorto-caval compression

 

As early as 13 weeks, the gravid uterus can compress the inferior vena cava when the woman is lying supine. Venous blood is diverted via collaterals, particularly the vertebral venous plexus to drain via the azygous system. Obstruction of the aorta occurs to a lesser degree but can result in decreased placental blood flow.

 

This is of great importance when positioning a patient on the operating table. During regional anaesthesia, the effects of aorto-caval compression will be exaggerated due to a lack of compensatory reflexes subsequent to the sympathetic blockade. This can lead to profound hypotension. It is usual to use either a wedge under the patient’s right flank, or to tilt the operating table to the left to counteract these effects. In extreme hypotension (or fetal compromise such as a bradycardia) the patient can be turned to the full left lateral position.

 

Haematology

 

The plasma volume increases up to 50% by term. Red cell volume also increases due to increased erythropoietin production, but not enough to prevent a dilutional anaemia occuring. There is a 15% drop in the measured haemoglobin. The blood viscosity is therefore also reduced which may slightly decrease cardiac work.

 

The platelet count tends to remain normal although there is probably an increase in their turnover. There is a steady rise in white cell count peaking during labour.

 

The total concentration of plasma protein is reduced due to the increase in plasma volume. This results in a drop in the colloid oncotic pressure, and may account for the oedema seen in pregnancy. Albumin levels drop to about 35 g/dl and the plasma cholinesterase levels decrease by about 25% reaching their nadir post-partum. Therefore, suxamethonium may have a slightly prolonged duration of action. The pharmacokinetics of protein bound drugs will be affected.

Coagulation is affected in pregnancy with an increase in most of the coagulation factors but also an increase in fibrinolytic activity. The result is a somewhat hyper-coagulable state. Thrombo-embolic disease is a very common condition and is one of the most important causes of maternal mortality in developed countries.

 

Renal Function

 

The renal system undergoes marked changes in function during pregnancy due to hormonal effects, the increased metabolic load of the fetus and also due to outflow obstruction of the ureters by the enlarging uterus. The glomerular filtration rate increases by up to 50% in pregnancy. As a result, the clearance of urea, uric acid and creatinine all increase and their plasma concentrations are lower in pregnancy.

 

Gastric Function

 

In the first trimester, hormonal changes may results in ‘morning sickness’. The most extreme form of this is labelled ‘hyper-emesis gravidarum’ and occasionally warrants admission to hospital for iv fluid resuscitation.

 

In pregnancy there is a relaxation of the lower oesophageal sphincter and an increase in intra-gastric pressure due to the expanding uterus. As a result of this, the symptoms of heartburn and reflux are common in pregnancy affecting up to 70% of women. There is also an increased risk of gastric regurgitation and aspiration during induction of general anaesthesia in the later stages of pregnancy. Pregnancy itself does not prolong gastric emptying time but labour pain and any opioids administered for the pain will do so.

 

Due to the combination of factors above, a rapid sequence induction is considered mandatory when inducing general anaesthesia in the third trimester and for 48 hours after delivery.

 

Musculo-skeletal system

 

The placenta produces relaxin, a hormone that causes widespread relaxation of ligaments. This results in widening and increased mobility of the pubis and sacro-iliac joints to allow passage of the fetus through the birth canal. Pain relating to these joints may occur during the later stages of pregnancy.

Due to the enlarging uterus, there is a compensatory increase in the lumbar lordosis. As a result, backache is a common complaint during pregnancy. Back pain in the post partum period is also very common and although there is no evidence that epidurals cause it, they are often blamed.

 

Neurological System

 

The minimal alveolar concentration (MAC) of volatile anaesthetics decreases during pregnancy. This may be secondary to the high levels of progesterone and possibly an increase in B endorphin levels. There is a similar increase in sensitivity to opioids, sedatives and local anaesthetics.

 

The effects of local anaesthetic drugs when used for neuraxial anaesthesia and analgesia are also enhanced secondary to mechanical factors within the epidural and subarachnoid space. As mentioned earlier, compression of the inferior vena cava results in diversion of blood through the vertebral venous plexus that lies within the epidural space. This causes the epidural veins to engorge and the volume of the epidural and sub-arachnoid space to decrease. Therefore, an identical volume of local anaesthetic will spread more extensively in the pregnant than in the non pregnant state. Cannulation of an epidural vein when performing epidural insertion (‘a bloody tap’) is also more common.

The constituents of cerebral spinal fluid (CSF) do not change during pregnancy but its volume is reduced due to compression from the epidural veins in the epidural space. The pressure of the CSF is therefore increased. Between contractions the pressure may be around 28 mm Hg but during painful contractions it may rise to as much as 70mmHg. It is therefore probably safer not to advance an epidural or spinal needle during contractions for risk of puncturing the dura and expulsion of CSF at high pressure.

Summary

 

Pregnancy brings about marked anatomical and physiological changes in the mother. These adaptations meet the increasing metabolic needs of the fetus and prepare the pregnant woman for parturition. The changes that occur in respiration, circulation, and digestion are the most relevant ones and every pregnant woman should be aware of these, so that she can tell the difference between what is normal and what is not. This will be very useful in detecting any problems during pregnancy well in advance, so that they can be managed appropriately.

 

 A number of changes occur in the cardiovascular system during pregnancy. The blood volume increases by up to 35% during the third trimester with a rise in the plasma volume and in the number of red blood cells. A greater increase in the plasma volume leads to a decreased red blood cell count, thereby lowering the hemoglobin and haematocrit values. The change in the plasma volume is triggered by increased production of a hormone aldosterone, which happens under the influence of placental lactogen, progesterone, and oestrogens. The alteration in the number of red blood cells is caused by increased activity of erythropoietin, a glycoprotein secreted by the kidneys that stimulates the production of red blood cells. During pregnancy, the cardiac output increases by both, an increase in the heart rate as well as the stroke volume. Despite this higher cardiac output, the blood pressure decreases because of a reduction in the peripheral vascular resistance.

 

 The respiratory system doesn't stay unaffected either. During pregnancy, oxygen consumption and carbon dioxide production increase by 60%. This is related to the metabolic needs of the fetus, uterus, and placenta as well as the increased cardiac and respiratory workload. The growing uterus displaces the diaphragm towards the head. Respiratory alkalosis is partially compensated for by increased excretion of bicarbonate by the kidneys, which is reflected by decreased serum bicarbonate levels. Increased levels of carbon dioxide in the mother's blood can lead to fetal acidosis. Vessel engorgement and increased vascularity of the upper respiratory tract cause it to swell up. This can make intubation difficult in pregnant women by reducing the diameter of the trachea, and increasing the chances of severe bleeding following trauma to the upper respiratory tract.

 

 Alterations in gastrointestinal function during pregnancy occur because the enlarged uterus displaces the stomach upwards and towards the left side of the diaphragm. In addition, the stomach is rotated 45 degrees to the right, and the angle between the axis of the stomach and the oesophagus changes. As a result of these changes, and also hormonal fluctuations, the gastric pressure increases and the lower oesophageal sphincter tone decreases. This causes regurgitation of gastric contents, giving rise to heartburn that is frequently reported in pregnant women. According to recent studies, the gastric pH and volume do not change during pregnancy.

 

 Early in pregnancy, due to an increase in the mucosal vasculature throughout the respiratory tract and excessive secretions in the nose, oropharynx, larynx, and trachea, women experience allergy-like symptoms, chronic colds, nasal congestion, voice changes, and mild breathlessness. In pregnancy, respiration is more abdominal than thoracic. The inspiratory capacity increases by 5 to 10% and the respiratory rate increases by two to three breaths per minute. The total lung capacity decreases by approximately 5%.

 

 During pregnancy, the kidneys enlarge by approximately 1 to 1.5 cm with a concomitant increase in weight. At full term, both kidneys are larger than in the non-pregnant state. Both the renal pelvis and the ureters dilate during pregnancy due to the relaxing effect of progesterone and the enlargement and rotation of the uterus to the right side. Since progesterone also decreases the bladder tone, there is an increase in the residual urine after emptying the bladder. There is also a loss of urinary sphincter control as pregnancy advances because the bladder is displaced upwards and flattened by the enlarging uterus. Reduction in the bladder capacity results in increased urinary frequency.

 

 Pregnancy leads to an increase in the vascularity of the reproductory organs. An increase in vaginal secretions produces a typical discharge. The uterus enlarges in size and becomes even more muscular. Its weight increases from approximately 70 gm in the non-pregnant state to more than 1000 gm at full term. Similarly, the volume of the uterine cavity grows from about 10 ml to approximately 5000 ml. The enlarged uterus causes an increase in the intra-abdominal pressure; and as the pregnancy progresses, the spine develops a forward curvature to compensate for the weight in the abdomen. Under the influence of progesterone and relaxin, the ligaments become more lax. There is also increased activity of PTH (parathyroid hormone), which leads to increased absorption of calcium from the intestines and decreased excretion of calcium by the kidneys.

 

The hormonal changes that occur from very early on in pregnancy cause a complex series of physiological and anatomical changes that affect every system of the body. To illustrate how all these changes may alter or affect anaesthetic management, it is useful to imagine performing a general anaesthetic for caesarean section and list some key points:

 

1. Careful attention to the assessment of the airway and any necessary preparation to deal with a potentially difficult airway in the pre operative period.

2. When positioning the patient on the table, remember to use either a left tilt of between 15 – 30 degrees on the table or a wedge under the right buttock to minimize aorto-caval compression.

3. Venous access if often easier due to engorgement of the venous system.

4. Pre oxygenation is essential and should be with a tight fitting mask for at least 3 minutes.

5. Rapid sequence induction with the application of cricoid pressure is mandatory. Intubation may be difficult and so adjuncts for difficult intubation should be available. The trained anaesthetic assistant should be careful when placing cricoid pressure if there is left tilt on the operating table as the temptation is to place the cricoid pressure straight down thus distorting the view at laryngoscopy.

6. Once the airway is secured, ventilation should be aimed to keep the PCO2 in the normal range for pregnancy.

7. The MAC of volatile anaesthetic is slightly reduced.

8. Volatile agents cause relaxation of the uterus (uterine atony) and may result in haemorrhage after delivery of the fetus.

9. There is decreased sensitivity to endogenous and exogenous catecholamines and so if vasopressors are required to maintain adequate blood pressure, the amounts needed may be greater.

10. Extubation should be done with the patient awake and on their side to reduce the risk of aspiration of gastric contents.

 

Exercise and Pregnancy

In the absence of complications, pregnant women should continue aerobic and strength training exercise for the duration of gestation.

 

Exercising While Pregnant

A strong, healthy woman will generally have a good pregnancy outcome.  Physicians recommend moderate exercise during pregnancy, including strength-training.

 

•Moderate aerobic exercise and strength training improve the health of pregnant women while having no adverse consequences on the developing fetus.

 

•A variety of exercise activities are appropriate, with the exception of those with a high risk for abdominal trauma, such as horseback riding, skiing, soccer, or hockey.

 

•Contraindications of exercise include: vaginal bleeding, dyspnea before exertion, dizziness, headache, chest pain, muscle weakness, preterm labor, decreased fetal movement, amniotic fluid leakage, and calf pain or swelling (to rule out thrombophlebitis).

 

contraindication

A factor or symptom which makes the prescribed treatment inadvisable.

 

strength-conditioning

Strength training is the use of resistance to muscular contraction to build the strength, anaerobic endurance, and size of skeletal muscles. There are many different methods of strength training, the most common being the use of gravity or elastic/hydraulic forces to oppose muscle contraction.